Add configurable sideload

Instead of having to manually change the `performSideload` variable on
all files, you'll only have to add a new config in blueprints.js file.
Example:

// config/blueprints.js
module.exports.blueprints = {
    // existing configuration
    // ...

    ember: {
        sideload: true
    }
}
Merge pull request #9 from sergiolepore/add/configurable-sideload

Add configurable sideload